Network Layer

First end-to-end^[End-to-end: the process of sending data from the originating source to the final destination] layer in the OSI reference model, responsible for delivery of data.

The Internet uses the Network layer

Addressing

every network interface on every host is intended to have a unique address

• hosts may change over time
• addressable $\ne$ reachable : firewalls

Fragmentation

MTU(Maximum transmission unit)

• Each link layer technology has a maximum packet size Example: Ethernet’s default MTU is 1500 bytes IPv4 Fragmentation
• If a packet exceeds the MTU, IPv4 routers will fragment it (break it down into smaller packets that are suitable for transmission)
• MF Bit (More Fragments)
  • Set if more fragments follow
  • Use the fragment offset and fragment identifier to reconstruct the original packet
• DF Bit (Don’t Fragment)
  • Set if routers should not fragment the packet
  • If a packet exceeds the MTU and the DF bit is set, the router must discard it IPv6 Fragmentation
• does not support fragmentation
• Fragmentation is hard to implement on high speed link
• Following the end-to-end principle, the responsibility is shifted to sending and receiving hosts

Loop Protection

to limit to the number of routers a packet can pass through before it is discarded. Why - Stops packets circling forever if a network problem causes a loop Packets include a forwarding limit:

• Set to a non-zero value when the packet is sent (typically 64 or 128)
• Each router that forwards the packet reduces this value by 1
• If zero is reached, packet is discarded

Header

An IP header is a segment of data at the beginning of an IP packet.

• contains important information required for routing and delivery of the packet across the network
• includes several fields that describe the packet’s source, destination and other essential details.

checksum

IPv4 header contains a checksum to detect transmission errors

• Protects the IP header only, not the payload data protected

Transport layer protocol identifier

• Network layer packets include the data from the transport layer as payload
• Must identify what transport layer protocol is used, to pass the data to the correct upper-layer protocol: (managed by IANA^[IANA (Internet Assigned Numbers Authority)])
  • TCP = Protocol Number 6
  • UDP = 17
  • DCCP = 33
  • ICMP = 1

IP Addresses

a unique identifier assigned to each device connected to a computer network that uses the Internet Protocol for communication. Addresses can denote host identity

• Addresses can denote host identity
• address can indicate the location at which a host attaches to the network

characteristics:

• They specify location of a network interface
• They are allocated hierarchically Domain names are a separate Application Layer namespace.

Structure

Addresses are split into two parts:

• Network
  • Netmask - describes the number of bits in the network part
• Host
  • network - itself has the address with the host part equal to Zero
  • broadcast address - for a network has all bits of host part equal to One A host with several network interfaces will have one IP addresses per interface

IP Address Management is managed by IANA^[IANA (Internet Assigned Numbers Authority)] who administers the pool of unallocated addresses and has all available addresses to RIR’s for IPv4.

Classification

• Traditional Allocation(classful Addressing)
  • Class A
    • /8 network
    • ~16 million addresses
  • Class B
    • /16 network
    • 65,356 addresses
  • Class C
    • /24 network
    • 26 addresses
• Modern Allocation(classless Addressing)
  • Arbitrary Length Netmask: Allowed since 1993